1. Field of the Invention
The present invention relates to an organic luminescent compound, and more particularly, to a new luminescent compound that contains at least two phenylcarbazole derivatives in a molecule and can be used in an emitting layer and/or a hole transporting layer of an organic electroluminescence (EL) device, and organic EL device using the compound.
2. Description of the Related Art
Electroluminescent (EL) devices known as a self-luminous display have the advantages of large viewing angle, high contrast property, and short response time. EL devices can be classified according to the material composing their emitting layer into either inorganic EL devices or organic EL devices. Organic EL devices have the advantages of higher luminance, lower driving voltage, shorter response time, and the ability to display a wider range of colors, over inorganic EL devices.
A general organic EL device includes an anode on the top surface of a substrate, with a hole transporting layer, an emitting layer, an electron transporting layer, and a cathode formed in sequence on the anode, wherein the hole transporting layer, the emitting layer, and the electron transporting layer are thin films made of organic compounds.
Organic EL devices operate according to the following principles. When a voltage is applied across the anode and the cathode, holes injected from the anode migrate via the hole transporting layer into the emitting layer. Electrons injected from the cathode migrate via the electron transporting layer into the emitting layer and combine with the holes therein to generate excitons. When the excitons transit from excited state to base state, fluorescent molecules in the emitting layer emit light to form visible images. Luminescence resulting from a transition from a singlet state (S1) to a base state (S0) is referred to as “fluorescence”, and luminescence resulting from a transition from a triplet state (T1) to a base state (S0) is referred to as “phosphorescence”. However, only 25% of singlet state excitons can be utilized for fluorescence, and the luminescence efficiency resulting from fluorescence is limited. Meanwhile, 75% of triplet state excitons and 25% of singlet state excitons can be utilized for phosphorescence, theoretically a 100% internal quantum efficiency can be achieved.
An organic EL device with superior green and red luminescence efficiency is disclosed in Nature 750 (Vol. 75, 2000). The organic EL device is manufactured using phosphorescent dyes Ir(ppy)3 and PtOEP, which respectively have heavy elements Ir and Pt with strong spin-orbit binding energy at their center, as dopants and CBP (4,4′-N,N′-dicarbazole-biphenyl) as a host to induce luminescence from phosphorescence in triplet state. However, the lifetime of the organic EL device is short at 150 hours or less, and the organic EL device is not suitable for commercial use. The CBP having a low glass transition temperature of 110° C. or less and being susceptible to crystallization accounts for such a short lifetime of the organic EL device.